The present invention relates to electro-optical imaging systems and, more particularly, to video processing apparatus for solid state imaging systems.
Electro-optical imaging sensors are roughly divided into camera-tubes contained within evacuated envelopes and solid state imaging sensors in which a charge pattern is created by the impingement of light on a solid state matrix array. One type of solid state imaging sensor, which forms the environment with which the present invention is employed, is commonly known as a charge-injection device (CID). The principles underlying charge-injection device imagers are detailed in U.S. Pat. Nos. 3,805,062; 3,949,162; 4,000,418; 4,011,441 and 4,011,442, the disclosures of which are herein incorporated by reference.
In brief, a charge-injection device employs a silicon substrate having orthogonal row and column conductors thereon which are insulated both from the substrate and from each other. Each intersection of a row conductor with a column conductor provides two storage locations, one under the row conductor and the other under the column conductor, within which charges liberated from the silicon substrate by incident radiation may be stored by the application of appropriate voltages. The stored charges, when appropriately read out, form the video signal.
Using an appropriately doped silicon substrate such as, for example, an n-type semiconductor, a negative voltage applied to a row or column conductor is effective to produce a depletion region forming a potential well thereunder. The potential well functions as a capacitor to collect the charges liberated by incident radiation. Although mutually insulated, the potential wells under the row and column conductors at an intersection thereof are so closely coupled that charges may be transferred back and forth therebetween without loss of stored charge. Whichever one of the row and column conductors is maintained at the more negative potential captures all of the charge from the one maintained at a less negative potential. In order to transfer the charge from beneath one conductor to beneath the other conductor, the voltage on the conductor originally having the larger negative voltage is reduced to a value less than the negative voltage on the originally less negative conductor. Equivalently, the negative voltage on the previously less negative conductor may be increased until it exceeds the negative voltage on the first-mentioned conductor.
In one technique described in the referenced patents, at all times except during the reading-out process, the row conductors are maintained more negative than the column conductors. The liberated charges are therefore totally contained under the row conductors. In preparation for reading out a row, the row voltage is raised until it attains a less-negative voltage intermediate the column voltage and ground. This transfers all of the accumulated charges simultaneously in the selected row from beneath all of the row conductors to beneath their respective column conductors. The negative voltages on the column conductors are then increased one at a time in sequence to a less negative voltage than the selected row conductor. The less negative voltage may conveniently be zero volts. As the voltage on each column conductor is increased to zero, the charge stored thereunder flows back beneath its associated row conductor within the row being read out. The flow of charges in the row conductor occasioned by the transfer of charge from each column conductor is sensed to produce the output video signal. It should be noted that, since the only column conductors which contain charges are those in the selected row, the voltage sequence on the column conductors is ignored by all storage locations except those in the selected row.
The readout sequence described above is non-destructive; that is, at the end of reading the stored charges in a row, the charges, although they have been transferred first from beneath the row conductors to beneath the column conductors and then have been sequentially transferred back again, remain in their original locations, undiminished. If the original voltages are restored on the row and column conductors, continued integration of incoming radiation without erasure of the previously stored charges may be performed. This is especially useful in low-light-level applications. In normal imaging applications, it is useful to erase the stored charges in a row just after it is read out so that a new charge pattern may be integrated until the next time the row is scheduled for readout. The charges in a row are readily cancelled or erased by raising the selected row voltage to zero while the column voltages are also at zero. This injects sufficient charges into the storage locations to cancel any charge pattern which they may have acquired, and hence the name "charge-injection device".
Noise is a problem in all imaging devices. The type of noise and its severity varies with the type of imaging device and with its required peripheral equipment. I have discovered that charge-injection imaging devices suffer from two sources of noise giving rise to pattern noise; namely, switching noise and capacitance variation noise.
The magnitude of the video output signal of a charge-injection device is usually a small fraction of the magnitude of the column-select signal voltage which is applied to the column conductors. The mutually insulated row and column conductors function as small capacitors which couple a portion of the column-select voltage for superposition onto the video signal on the selected row conductor on which the video signal is transmitted to external circuits. For common television signal rates, the column-select signal has frequency components in the range of 3.5 MHz, 7 MHz and higher. The 3.5 MHz components, in particular, produce a pattern noise in the video signal which is objectionable when large values of video gain are employed. Simple filtering of the video to remove the 3.5 MHz component is not desirable since such filtering would also remove significant video information existing in the vicinity of this frequency.
Capacitance variation noise is produced by slight differences in the values of capacitances of the cells making up the matrix. As a consequence, uniform illumination of all of the cells induces the storage of slightly different amounts of charge. In effect, the differences in cell capacitance produces a video signal variation from cell to cell even when all of the cells are uniformly illuminated. When a non-uniform scene is imaged on the matrix, the pattern noise produced by the capacitance differences is essentially superimposed on the video representing the scene. This effect is particularly troublesome when high video gain is used in low-light-level applications.
The prior art has taken advantage of the fact that the capacitance pattern of corresponding cells in adjacent rows is similar. Two adjacent rows are simultaneously enabled and read out by the same sequence of voltages on the row and column conductors. The pattern noise from the immediately preceding row, which was erased at the end of its readout, is inverted and subtracted, cell-by-cell, from the output of the row containing the desired video information. Due to the similar capacitances of corresponding cells in adjacent rows, the inverted pattern noise from an erased row, subtracted from the video plus pattern noise from corresponding storage locations in the immediately following row, cancels a substantial portion of the pattern noise originating in charge variation. This technique has permitted the successful use of charge-injection imaging devices in applications where their small size and ruggedness are an advantage.
Even after cancelling pattern noise using adjacent-row noise residue, a small residue of pattern noise remains due to the fact that, although adjacent-row storage locations are very similar, they are not, in fact, exactly the same. Thus, in demanding imaging applications including, for example, low-light-level imaging in which high video gain is required, a reduced but still-visible pattern noise is present.
U.S. Pat. No. 4,079,423 discloses a technique in which the output from the same row before and after video erasure is used for pattern-noise cancellation. The video data of a row, accompanied by its pattern noise, is delayed for one horizontal interval (1H) and is then added to the inverted undelayed pattern noise from the same line from which the video information has been erased. Since the sources of both of these signals are identical, improved cancellation is achieved. Any residue of pattern noise which remains after cancellation is inverted in succeeding lines to provide visual cancellation of pattern noise.